Render semi-transparent volume with cycles
PNG images can't represent objects that emit light but not block any light, due to the way they represent transparency. For that reason, you must always have some amount of volume absorption for the emission to be visible when saved as a PNG image.

If your volume has zero density, you could use the flame to drive the volume absorption density, adding just enough to keep the flame visible.

The render and compositing image representations and EXR images can represent such objects, but this information can't be preserved when displaying in the image editor or converting to PNG. This is one of the subtle differences between associated/premultiplied and unassociated/unpremultiplied alpha.